That sounds really awesome - unfortunately, that's not how it would end up working out.
If we were able to use sliders, or the web that you mentioned, within a few days, someone would math out the optimal setup for each module, and most likely, for entire loadouts. As such, this is what the majority of people who Engineer would do with every modification. You might think otherwise, but consider the Reforging system from World of Warcraft. It was put into the game for the express purpose of allowing players to customize their secondary stat distributions, and build their characters in a unique way. This was the vision, but that isn't what ended up happening. Optimal Reforging was quickly mathed out for every spec of every class, and soon after, a 3rd party add-on was created that did it for you automatically. Instead of individualization, it led to cookie-cutter builds (that were, in fact, optimal). Since it didn't meet the vision (and because of stat bloat), it was removed from the game. The same thing will happen here, too. This is half of the reason that the Engineers are RNG, and why they need to be. The other half of the reason is because unicorn parts are awesome, but they can only be done using an RNG system. If you could make those on demand (no matter how many extra mats it might require), then they are not special parts, they become "standard" upgrades, which we already have in the form of A-E.
What you wrote sounds great - tell me how what I wrote sounds.
Riôt